1. Field of the Invention
The present invention relates typically to an acousto-optic device and a fabrication method of an acousto-optic device, and more particularly to an acousto-optic device suitable for use with an acousto-optic tunable filter in a reception section of a wavelength division multiplexing (WDM) transmission node for demultiplexing a signal light having a desired wavelength from wavelength division multiplexed light (WDM light) including a plurality of multiplexed signal lights individually having different wavelengths and to a fabrication method of the acousto-optic device.
2. Description of the Related Art
In recent years, in order to achieve expansion of the transmission capacity and increase of the transmission distance, a WDM transmission system has been popularized. In an add/drop node (optical transmission apparatus) provided in a network which forms the WDM transmission system, a dropping (branching) process and an adding (inserting) process are performed for an inputted WDM light. For example, a plurality of signal lights of different channels included in the inputted WDM light are dropped and signal lights having desired wavelengths from among the wavelengths included in the local signal lights are selected and added and outputted to a WDM transmission line.
FIG. 17 is a diagrammatic view illustrating an add/drop (Add and Drop) function. An add/drop node 90 shown in FIG. 17 selects and drops one or more single wavelength lights (single lights) included in a WDM light from a repeating node 90a, and transmits the single lights to an optical network 90d. Further, the add/drop node 90 passes those single wavelength lights which are not to be dropped to a repeating node 90b therethrough, and adds one or more single wavelength lights from an optical network 90c to free wavelengths of the WDM light. This adding and dropping process is performed by an acousto-optic tunable filter (AOTF) 89 provided in the add/drop node 90.
The acousto-optic tunable filter 89 is an acousto-optic device and includes an input port 88a to which WDM light is to be inputted, an add port 88b to which local signal lights are to be inputted, a non-selection port 88c for outputting single wavelength lights which are not dropped, and a selection port 88d for outputting dropped single wavelength lights.
Consequently, the acousto-optic tunable filter 89 functions as a variable wavelength filter having both functions of a band-block filter and a band-pass filter. Further, this function is implemented in one chip, and is given attention also from the point of view of the cost.
As a material for a substrate of the acousto-optic tunable filter 89, LiNbO.sub.3 (lithium niobate) is commonly used. However, in polar crystal of LiNbO.sub.3 or the like, electric charge (polarization electrode) by spontaneous polarization is generated by a temperature variation or crystal distortion (in a normal state, such spontaneous polarization is neutralized by space charge).
Then, when a TE (Transverse Electric) wave and a TM (Transverse Magnetic) wave propagate along the surface of the substrate and the inside of the substrate spaced a little from the surface of the substrate, the TE wave is converted into a TM wave while the TM wave is converted into a TE wave.
Conventionally, methods for miniaturization, integration, reduction of the power consumption and the cost, improvement of the stability of operation of an acousto-optic tunable filter which can exhibit the add/drop function have been proposed (refer to Patent Documents 1 and 2).
A waveguide type optical device disclosed in the Patent Document 1 includes a substrate having an electro-optical effect, an optical wavelength, a dielectric layer on the optical waveguide, electrodes, and a conductive film layer which covers an exposed surface of the dielectric layer. Further, in the waveguide type optical device, electrical connection by the conductive film layer disposed between electrodes adjacent each other is cut at least along the direction of the waveguide, and part of the dielectric layer is exposed at the cut portion. This stabilizes the electrode characteristic of the waveguide type optical device and promotes the reliability thereof.
An electro-optic device disclosed in the Patent Document 2 includes a LiNbO.sub.3 substrate including an optical waveguide formed on the upper side surface thereof, a buffer layer including a BCB dielectric material directly formed on the upper side surface of the LiNbO.sub.3 substrate and having a thickness greater than 7000 angstrom, and an electrode formed on the upper side surface of the buffer layer, arranged to allow the electrode to receive an RF signal (Radio Frequency signal), so that the RF signal introduces an electric field to the optical waveguide. Consequently, an optical device having an improved temperature stability and including a discharging path for moving charge can be obtained, and formation of a strong electric field along the +z plane and the −z plane of z-cut LiNbO.sub.3 crystal can be prevented.
Patent Document 1 Japanese Patent Laid-Open No. HEI 8-54589
Patent Document 2 Published Japanese Translation of a PCT Application, No. 2002-542511
However, charge generated in the z direction of the acousto-optic tunable filter has an influence on the refractive index of the substrate, and makes a factor to vary the filter characteristic (band pass characteristic) of the acousto-optic tunable filter.
Further, if electric charge generated by spontaneous polarization is accumulated in the electrodes (comb type electrode) provided for exciting a surface acoustic wave and is discharged between the electrodes, then there is the possibility that the electrodes or the substrate may be damaged.